Outboard Oil Ratios

Author: Stuart Buckingham   Date Posted:25 July 2017 

Blog Outboard oil ratiosAnyone who owns, or has owned, a two stroke outboard would have asked themselves the question; how much oil do I mix with my fuel? 100:1, 50:1 or 25:1? It is one of the most asked questions for those required to pre-mix their oil and fuel together. Below I have listed the oil ratios recommended by manufacturers and a few tips based on my experience.

100:1 - Mix 50ml of oil per 5ltrs of fuel. Recommended by Yamaha and Suzuki for most of their small two stroke engines up to about 30hp, this ratio requires the least amount of TCW3 two stroke oil. The result of using this ratio (less oil) is reduced spark plug fouling and less smoke. For older Suzuki outboards, pre-1997, I'd recommend running 50:1 as the information I've provided about is for current model two strokes.
Note: For commercial applications Suzuki recommends 50:1.

50:1 - Mix 100ml of oil per 5ltrs of fuel. Recommended by Mercury, Mariner, Tohatsu, Johnson and Evinrude this is my preferred ratio for all outboards including Yamaha and Suzuki. You can confidently use this ratio for any horse power and just about all year models. Whilst this ratio might see spark plugs fouling with extended use at lower RPM (such as trolling), in my opinion, it provides the best amount of lubrication for your outboard.

25:1 - Mix 200ml of oil per 5ltrs of fuel. This mixture is very rich and is usually recommended for running in brand new two stroke outboards and when running in rebuilt or reconditioned engines. If you have a rebuilt or reconditioned oil injected two stroke outboard (excluding Evinrude ETEC or Mercury Optimax) then you'll just need to add 50:1 to your fuel and let the oil injection system add the rest of the oil to make up the balance to get to 25:1. Most technicians will have their own opinion or view on how long you should run the 25:1 ratio for but, I always recommend between 8 and 10 hours. Unfortunately, you will foul plugs at this ratio and leave a very impressive smoke plume on engine start up but, outboards that have been rebuilt or reconditioned need this extra oil while the piston rings bed into the bores.

Oil ratios can be confusing and daunting. The last thing anyone wants to do is not use enough oil and damage their pride and joy. Hopefully, the above answers any questions you may have and gives you a better understanding of what ratio your outboard requires.

Shop for Penrite outboard oils & lubricants here

Comments (7)

No power under load

By: on 18 April 2021
We mixed 25:1 for old 70hp evenrude motor ran ok. On trailer reved alright and ideld ok but on water motor kept dieing and cutting out when starting to load power up could this problem just be too much oil mix

Outboard Spares Response
Hi, This could be a number of issues such as faulty fuel pump, carburetors needing cleaning, ignition issue, etc. I think running 25:1 will not be the issue with this engine.

40 HP mariner yamaha 1979 model

By: on 29 January 2021
Hi guys please assist. I have these two motors however do not know the fuel mixture as some old small motors use to run a 100:1 ratios where as most are 50:1. How will I know. I also do know that fully synthetic has its own mixture like 75:1.

Outboard Spares Response
Hi, If I owned one of these engines I'd be running 50:1 with a decent TCW3 2 stroke engine oil designed for pre-mix. If you're doing a lot of idling and slow trolling you could reduce the ratio a bit if you found you are fouling plugs but honestly I think 50:1 should be fine. Cheers

2 stroke oil

By: on 1 August 2020
A Great Rule Of Thumb on 2 stroke oil small engines = less oil 100-1 / 50-1 Engines before 2000 and beyond large or small 50-1 new engines 25-1 / 20 to 40 liters

Oil suitability

By: on 12 March 2020
Are you able to use the same oil as for aircooled engines such as chainsaws etc

Outboard Spares Response
Hmmm. Good question. Outboard oils are designed for water-cooled engines. I would not suggest it. Cheers


By: on 10 August 2018
Thanksamundo for the post.Really thank you! Awesome.

2 Stroke oil

By: on 25 July 2017
Is the said ratios needed for large hp 2 strokes . Year 2000 forward . As Most are oil injected anyway . What's the reason for adding oil to the fuel mix if the motor is running fine . No rebuild ect . Dose it save oil being injected .

Outboard Spares Response
If your outboard is oil injected, and hasn't been rebuilt, then you won't need to add any oil to your fuel. But adding oil to your fuel will not change the amount injected by the engines oil injection system.


By: on 25 July 2017
Simple divide 1000ml or 1 litre of petrol by ratio = the required amount of oil per litre of fuel i.e 1000 / 50:1 = 20ml of oil per litre of fuel Or I.e 1000 ml of petrol divide by 100:1 = 10mls of oil per litre

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